1. Technical Field
The invention relates to an ignition controller that controls ignition of a discharge lamp discharging and emitting light between a pair of electrodes provided therein, a light source, a projector, and an ignition control method for the discharge lamp.
2. Related Art
A discharge lamp such as an extra-high pressure mercury lamp including a light-emitting tube having a light-emitting portion provided with a discharge space in which a pair of electrodes are disposed, and a pair of sealing portions which extend away from each other sandwiching the light-emitting portion and are provided with an electrode-connecting wire has been traditionally known. In the discharge space of such discharge lamp, light-emitting materials such as mercury and rare gas are sealed. The mercury is adhered to easier-to-be-cooled one of the pair of the electrodes disposed in the discharge space when the discharge lamp is turned off. When a current is applied to such electrodes while the mercury is not evaporated due to insufficient temperature rise, the current scarcely flows between the electrodes since impedance between the electrodes is high. On the other hand, when the mercury is evaporated after the temperature is sufficiently raised, the current satisfactorily flows between the electrodes since the impedance between the electrodes lowers. Therefore, high-efficiency of temperature increase in the electrodes leads to startup time reduction for the discharge lamp.
As an ignition controller for controlling ignition of such discharge lamp, a high-pressure discharge lamp ignition device has been known, in which: an alternating current rectangular wave voltage is applied on startup of the discharge lamp; a direct current voltage is applied during a transition from a glow discharge to an arc discharge; and another alternating current rectangular wave voltage is applied during a steady lighting period (for example, see Document 1: JP-A-2004-39391).
In the high-pressure discharge lamp ignition device disclosed in Document 1, a high-frequency alternating current voltage is applied on startup of the discharge lamp for applying a current to each electrode corresponding to polarity reverse. Then, the direct current voltage is applied so that temperature of each electrode adequately increases. Further, another alternating current voltage is applied again so that lighting goes into a steady state.
However, in the high-pressure discharge lamp ignition device disclosed in Document 1, it is difficult to apply the current rapidly to the electrodes as in actuation by the direct current, since the alternating current voltage is applied to the discharge lamp when the lamp is actuated. Consequently, a current sufficient for raising the temperature cannot be applied to the electrodes. Therefore, the above-described discharge lamp tends to generate only one-way current conduction between the electrode to which mercury is not adhered and the other electrode to which the mercury is adhered (asymmetrical current [or half-wave current] drive). Such asymmetrical current may damage the electrodes. Additionally, when the temperature of the electrodes does not rapidly increase, the transition from the glow discharge period to the arc discharge period delays. This also may cause the damage to the electrodes.